WASHINGTON — Iridium announced Jan. 31 it has purchased an additional Falcon 9 launch from SpaceX that the satellite services company will share with a German-U.S. Earth science mission.

The additional launch, planned to take place by early 2018 from Vandenberg Air Force Base in California, will carry five Iridium Next satellites as well as the two satellites for the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, a joint project of NASA and the German Research Centre for Geosciences, known by the German acronym GFZ.

GRACE-FO is a successor to the GRACE mission, launched in 2002, to map the Earth’s gravity field to high precision, allowing scientists to track changes in large-scale water movement. GRACE-FO is similar to GRACE, but includes a laser tracking system designed to improve the precision of the spacecraft system’s gravity measurements.

GRACE-FO was originally slated to launch on a Dnepr rocket, a converted intercontinental ballistic missile, from Baikonur, Kazakhstan. GFZ arranged the launch under a contract with Moscow-based launch provider Kosmostras as part of its contribution to the overall mission. However, the Dnepr has not launched since March 2015 because of difficulties winning Russian government approval for additional launches of the Ukrainian-built rocket.

Iridium has also been a victim of Dnepr delays. The company originally planned to launch its first two next-generation satellites on a Dnepr to test them in orbit before beginning large-scale deployment of the constellation on Falcon 9 launches. Iridium ultimately decided to start with the Falcon 9, successfully launching its first 10 satellites Jan. 14.

“This is a very smart way to get additional Iridium Next satellites into orbit,” said Iridium Chief Executive Matt Desch in a statement. “This launch provides added resiliency to our network for not much more than we had planned originally to launch 72 satellites, including two with Kosmotras.”

Iridium said in the statement that the additional cost of the shared Falcon 9 launch is “immaterial” because of the “avoidance of unspent amounts” on money under the original plans for a Dnepr. Iridium also plans to adjust its plans for launching its remaining satellites and their arrangement in their various orbital planes so that the five additional ones can be launched directly into their planned operational orbits.

The rideshare launch does bring with it additional complexity in terms of both integrating the two different sets of satellites and launching them into separate orbits. While the Iridium satellites fly in orbits at 780 kilometers high with an inclination of 86.4 degrees, the GRACE-FO spacecraft will operate in an orbit 490 kilometers high at an inclination of 89 degrees.

Iridium did not disclose the terms of the new contract, including what portion of the launch costs — a Falcon 9 has a list price of $62 million — it will pay. Iridium noted in the statement that GFZ “has been a great business partner throughout this process.” It added it would also consider launching additional satellites with Kosmotras “once approvals are available.”